This invention relates generally to dc-to-dc converters which convert one unidirectional voltage into another and, more particularly, to a dc-to-dc converter relying on feedback control for such conversion.
The dc-to-dc converter of the type herein under consideration generally comprises a transformer having a primary winding connected to a dc power supply, and a secondary winding connected to a rectifying and smoothing circuit. Connected in series with the transformer primary, a semiconductor switching device such as a field-effect transistor is driven on and off under the direction of a switch control circuit which is responsive to outputs from both an output voltage detector circuit and a circuit that detects the current flowing through the switching device.
There are the following three known methods of driving the switching device:
1. Ringing Choke Converter (RCC):
The switching frequency grows higher with less power requirement by the load.
2. Pulse Duration Modulation (PDM) or Pulse Width Modulation (PWM):
The switching device is driven at a constant rate (repetition frequency) but closed for less periods of time with less power requirement by the load.
3. Fixed Nonconducting Period:
The switching device is open for a constant period of time and closed for a period depending upon the load. Since one switching cycle is the sum of one conducting period and one nonconducting period, the switching frequency is subject to les change than by the RCC method.
The RCC and fixed nonconducting period methods are alike in that the switching frequency rises under light load, as when the converter is in standby mode. With the switching device turned on and off a greater number of times for a given amount of power fed to the load, the converters operating by these known methods have incurred unnecessarily high switching losses under light load.
The switching frequency is set as high as, say, 100 kilohertz according to the PDM switching method, with a view to less transformer loss under normal load as well as to the need for a smaller transformer. The switching device is therefore driven at that high frequency under light load, too. Thus the PDM switching method is also poor in efficiency under light load.
Japanese Unexamined Patent Publication No. 9-140128 provide a solution to the problems possessed by all of the above more conventional switching methods, suggesting use of different switching frequencies for normal and light load. The switching frequency is made lower under light load for attainment of higher efficiency by reduction of switchings per unit length of time.
Although this known suggestion seems the most reasonable solution to the problem of how to improve converter efficiency under light load, difficulties have been encountered in automatically changing the switching frequency according to the load. Automatically changing the switching frequency requires, of course, an accurate detection of the load magnitude. As far as the applicant is aware, however, no circuit has yet been proposed that can accurately detect the load magnitude but that is simple enough in construction to be incorporated with a dc-to-dc converter without substantially adding to its manufacturing cost.